The importance of de novo synthesis of amino acids by the human gut will be estimated by using [2H4]lysine, [15N]ammonium chloride or [15N2] urea, and [18O]urea under an adequate dietary protein intake in young adult healthy subjects with ileostomies. Additionally, by using ammonium chloride and urea we will be able to determine their respective importance as a nitrogen source for amino acid synthesis by the intestinal microflora.